Multi-bit resistance-switching memory cell

ABSTRACT

A non-volatile storage apparatus comprises a set of Y lines, a set of X lines and a plurality of memory cells in communication with the set of X lines and the set of Y lines. Each memory cell of the plurality of memory cells includes a resistance element in a static resistance condition and two or more reversible resistance-switching elements. The resistance element in the static resistance condition and the two or more reversible resistance-switching elements are connected to different Y lines of the set of Y lines. The resistance element in the low resistance state and the two or more reversible resistance-switching elements are connected to a common X line of the set of X lines. One or multiple bits of data are programmed into a particular memory cell of the plurality of memory cells by causing current flow between Y lines connected to the particular memory cell.

This application is a divisional application of U.S. patent applicationSer. No. 12/418,212, “Multi-Bit Resistance-Switching Memory Cell,” byScheuerlein, filed on Apr. 3, 2009, which is incorporated herein byreference in its entirety.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No.13/396,501, “Multi-Bit Resistance-Switching Memory Cell,” byScheuerlein, filed Feb. 14, 2012 the same day as the presentapplication, which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field

The present invention relates to technology for data storage.

2. Description of the Related Art

A variety of materials show reversible resistance-switching behavior.These materials include chalcogenides, carbon polymers, graphiticcarbon, carbon nano tubes, perovskites, and certain metal oxides andnitrides. Specifically, there are metal oxides and nitrides whichinclude only one metal and exhibit reliable resistance switchingbehavior. This group includes, for example, NiO, Nb₂O₅, TiO₂, HfO₂,Al₂O₃, MgO_(x), CrO₂, VO, BN, and ALN, as described by Pagnia andSotnick in “Bistable Switching in Electroformed Metal-Insulator-MetalDevice,” Phys. Stat. Sol. (A) 108, 11-65 (1988). A layer of one of thesematerials may be formed in an initial state, for example a relativelylow-resistance state. Upon application of sufficient voltage, thematerial switches to a stable high-resistance state. This resistanceswitching is reversible such that subsequent application of anappropriate current or voltage can serve to return theresistance-switching material to a stable low-resistance state. Thisconversion can be repeated many times. For some materials, the initialstate is high-resistance rather than low-resistance.

These reversible resistance-switching materials are of interest for usein nonvolatile memory arrays. One resistance state may correspond to adata “0,” for example, while the other resistance state corresponds to adata “1.” Some of these materials may have more than two stableresistance states.

Non-volatile memories formed from reversible resistance-switchingelements are known. For example, U.S. Patent Application Publication2006/0250836, filed May 9, 2005 and titled “REWRITEABLE MEMORY CELLCOMPRISING A DIODE AND A RESISTANCE-SWITCHING MATERIAL,” which is herebyincorporated by reference herein in its entirety, describes arewriteable non-volatile memory cell that includes a diode coupled inseries with a reversible resistance-switching material such as a metaloxide or metal nitride.

However, operating memory devices that employ reversibleresistance-switching materials is difficult.

SUMMARY

A storage system is described that uses reversible resistance-switchingelements. Various circuits and methods are disclosed for controlling thereading, setting and resetting of the resistance for the reversibleresistance-switching elements.

One embodiment of a non-volatile storage apparatus comprises a set of Ylines, a set of X lines and a plurality of memory cells in communicationwith the set of X lines and the set of Y lines. Each memory cell of theplurality of memory cells includes a resistance element in a staticresistance condition and two or more reversible resistance-switchingelements. The resistance element in the static resistance condition andthe two or more reversible resistance-switching elements are connectedto different Y lines of the set of Y lines. The resistance element inthe static resistance condition and the two or more reversibleresistance-switching elements are connected to a common X line of theset of X lines.

One embodiment of a non-volatile storage apparatus comprises an X line;a first Y line; a second Y line; a third Y line; a semiconductor regionof a first type running along the X line; first resistance-switchingmaterial and a first semiconductor region of a second type between thefirst Y line and the semiconductor region of the first type; secondresistance-switching material and a second semiconductor region of thesecond type between the second Y line and the semiconductor region ofthe first type; first static resistance material and a thirdsemiconductor region of the second type between the third Y line and thesemiconductor region of the first type; and control circuitry incommunication with the X line, the first Y line, the second Y line andthe third Y line. The first semiconductor region of the second type isadjacent to the semiconductor region of the first type. The secondsemiconductor region of the second type is adjacent to the semiconductorregion of the first type. The third semiconductor region of the secondtype is adjacent to the semiconductor region of the first type. Thecontrol circuitry changes the non-volatile memory cell to a first stateby causing a first current to flow from the third Y line to the first Yline through the first static resistance material and the firstresistance-switching material.

One embodiment of a method of programming a non-volatile storage systemcomprises programming multiple sets of data into different multi-bitmemory cells of a plurality of multi-bit memory cells. Each set of dataincludes more than one bit of data. Each multi-bit memory cell includesa resistance element in a conductive condition and two or morereversible resistance-switching elements. The resistance element in theconductivecondition and the two or more reversible resistance-switchingelements are connected to different Y lines. The resistance element inthe conductivecondition and the two or more reversibleresistance-switching elements are connected to a common X line. For atleast a subset of the multi-bit memory cells, the programming comprisespassing a first current between different Y lines.

One embodiment of a method of programming a non-volatile storage systemcomprises programming multiple bits of data into a single memory cell.The single memory cell includes a resistance element statically in aconductivecondition that is connected to a particular Y line and acommon X line. The single memory cell includes a first reversibleresistance-switching element connected to a first Y line and a secondreversible resistance-switching element connected to a second Y line.The first reversible resistance-switching element and the secondreversible resistance-switching element are connected to the common Xline. The programming comprises a first operation that includes passinga first current between the particular Y line and the first Y line tocause the first reversible resistance-switching element to change stateto a low resistance state. The first current passes through theresistance element in the conductivecondition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified prospective view of one embodiment of a memorycell with a reversible resistance-switching element.

FIG. 2 is a simplified side view of a portion of a three dimensionalmemory array formed from a plurality of the memory cells of FIG. 1.

FIG. 3 is a block diagram of one embodiment of a memory system that canbe formed on one or more integrated circuits.

FIG. 4 is a perspective view of a portion of a three dimensional memoryarray.

FIG. 5 is a perspective view of a portion of a three dimensional memoryarray.

FIG. 6 is a graph depicting I-V characteristics of a reversibleresistance-switching element.

FIG. 6A is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a Read operation.

FIG. 7 is a flow chart describing one embodiment of a process forperforming a SET of the resistance of one example of a memory cell.

FIG. 8 is a schematic diagram of a portion of one embodiment of a memoryarray during one embodiment of a SET operation.

FIG. 9 is a schematic diagram of a portion of one embodiment of a memoryarray during one embodiment of a SET operation.

FIG. 10 is a flow chart describing one embodiment of a process forperforming a RESET operation for one example of a memory cell.

FIG. 11 is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a RESET operation.

FIG. 12 is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a RESET operation.

FIG. 13 is a schematic diagram of a portion of one embodiment of amemory array.

FIG. 14 is a flow chart describing one embodiment of a process forperforming a Forming operation.

FIG. 15 is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a Forming operation.

FIG. 16 is a flow chart describing one embodiment of a process forperforming a SET operation for a memory array.

FIG. 17 is a flow chart describing one embodiment of a process forperforming a SET operation for a memory cell.

FIGS. 18-21 are schematic diagrams of a portion of one embodiment of amemory array during one embodiment of a SET operation.

FIG. 22 is a flow chart describing one embodiment of a process forperforming a RESET operation for a memory cell.

FIG. 23 is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a RESET operation.

FIG. 24 is a schematic diagram of a portion of one embodiment of amemory array.

FIG. 25 is a schematic diagram of a portion of one embodiment of amemory array.

FIG. 26 is a schematic diagram of a portion of one embodiment of amemory array.

FIG. 27 is a flow chart describing one embodiment of a process foroperating a memory array.

FIG. 28 is a flow chart describing one embodiment of a process foroperating a memory array.

FIG. 29 is a flow chart describing one embodiment of a process forperforming a Forming operation.

FIG. 30 is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a Forming operation.

FIG. 31 is a flow chart describing one embodiment of a process forperforming a SET operation.

FIG. 32A is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a SET operation.

FIG. 32B is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a SET operation.

FIG. 33 is a flow chart describing one embodiment of a process forperforming a RESET operation.

FIG. 34A is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a RESET operation.

FIG. 34B is a schematic diagram of a portion of one embodiment of amemory array during one embodiment of a RESET operation.

DETAILED DESCRIPTION

A memory system is provided that includes memory cells with one or morereversible resistivity-switching elements. Various circuits and methodsare disclosed for controlling the setting and resetting of theresistance for the reversible resistance-switching elements.

Memory Cell and System

FIG. 1 is a simplified perspective view of one embodiment of a memorycell 14, which includes one reversible resistance-switching elementcoupled in series with a steering element, between a first conductor 10and a second conductor 12. The steering element can take the form of anysuitable device exhibiting a nonlinear conduction currentcharacteristic, such as a diode.

The reversible resistance-switching element includes reversibleresistivity-switching material having a resistivity that may bereversibly switched between two or more states. For example, thereversible resistivity-switching material may be in an initialhigh-resistivity state upon fabrication that is switchable to alow-resistivity state upon application of a first voltage and/orcurrent. Application of a second voltage and/or current may return thereversible resistivity-switching material to the high-resistivity state.Alternatively, the reversible resistance-switching element may be in aninitial low-resistance state upon fabrication that is reversiblyswitchable to a high-resistance state upon application of theappropriate voltage(s) and/or current(s). When used in a memory cell,one resistance state may represent a binary “0” while another resistancestate may represent a binary “1.” However, more than two data/resistancestates may be used. Numerous reversible resistivity-switching materialsand operation of memory cells employing reversible resistance-switchingmaterials are described, for example, in U.S. Patent ApplicationPublication 2006/0250836, incorporated herein by reference.

In one embodiment, the process of switching the resistance from thehigh-resistivity state to the low-resistivity state is referred to as aSET operation. The process of switching the resistance from thelow-resistivity state to the high-resistivity state is referred to as aRESET operation. In one embodiment, the high-resistivity state isassociated with binary data “0” and the low-resistivity state isassociated with binary data “1.” In other embodiments, SETTING andRESETTING and/or the data encoding can be reversed.

In some embodiments, the reversible resistance-switching material may beformed from a metal oxide. Various different metal oxides can be used.In one example, nickel oxide is be used.

In at least one embodiment, through use of a selective depositionprocess, a nickel oxide layer may be used in a reversibleresistance-switching material without the nickel oxide layer beingetched. For example, a reversible resistance-switching element may beformed by employing a deposition process such as electroplating,electroless deposition, or the like, to selectively deposit anickel-containing layer only on conductive surfaces formed above asubstrate. In this manner, only the conductive surfaces on the substrateare patterned and/or etched (prior to deposition of thenickel-containing layer) and not the nickel-containing layer.

In at least one embodiment, the reversible resistance-switching materialincludes at least a portion of a nickel oxide layer formed byselectively depositing nickel and then oxidizing the nickel layer. Forexample, Ni, Ni_(x)P_(y) or another similar form of nickel may beselectively deposited using electroless deposition, electroplating or asimilar selective process, and then oxidized to form nickel oxide (e.g.,using rapid thermal oxidation or another oxidation process). In otherembodiments, nickel oxide itself may be selectively deposited. Forexample, an NiO-, NiO_(x)- or NiO_(x)P_(y)-containing layer may beselectively deposited above the steering element using a selectivedeposition process and then annealed and/or oxidized (if necessary).

Other materials may be selectively deposited, and then annealed and/oroxidized if necessary, in accordance with the present invention to formreversible resistivity-switching materials for use in memory cells. Forexample, a layer of Nb, Ta, V, Al, Ti, Co, cobalt-nickel alloy, etc.,may be selectively deposited, such as by electroplating, and oxidized toform a reversible resistivity-switching material.

More information about fabricating a memory cell using reversibleresistance-switching material can be found in U.S. patent applicationSer. No. 11/772,084, “Memory Cell That Employs A Selectively DepositedReversible Resistance Switching Element and Methods of Forming TheSame,” filed on Jun. 29, 2007, published as U.S. Patent App.2009/0001343, incorporated herein by reference in its entirety.

Conductors 10 and 12 are typically orthogonal to one another and formarray terminal lines for accessing an array of memory cells. The arrayterminal lines (also called array lines) at one layer may be termed Xlines. The array terminal lines at a vertically adjacent layer may betermed Y lines. A memory cell can be formed at the projectedintersection of each X line and each Y line, and connected between therespective intersecting X line and Y line. The terms “X line” and “Yline” are arbitrary labels and other terms can also be used. In oneembodiment, X lines will serve as word lines and Y lines will serve asbit lines. In another embodiment X lines will serve as bit lines and Ylines will serve as word lines. In other embodiments, X lines and Ylines will be used differently. In one example, conductor 10 of FIG. 1is a Y line and conductor 12 is an X line.

Conductors 10 and 12 include any suitable conductive material such astungsten, any appropriate metal, heavily doped semiconductor material, aconductive silicide, a conductive silicide-germanide, a conductivegermanide, or the like. In the embodiment of FIG. 1, conductors 10 and12 are rail-shaped and extend in different directions (e.g., insubstantially perpendicular directions). Other conductor shapes and/orconfigurations may be used. In some embodiments, barrier layers,adhesion layers, antireflection coatings and/or the like (not shown) maybe used with conductors 10 and 12 to improve device performance and/oraid in device fabrication.

FIG. 2 is a side cutaway view of a portion of an exemplary monolithicthree-dimensional memory array that can be used in one embodiment.However, other memory structures can be used in accordance with variousembodiments, including two-dimensional memory structures manufacturedon, above, or within a semiconductor substrate. Both the X line and Yline layers are shared between memory cells in vertically adjacentlayers the structure depicted in FIG. 2. This configuration is oftenreferred to as a fully mirrored structure. A plurality of substantiallyparallel and coplanar conductors form a first set of Y lines 162 at afirst memory level L0. Memory cells 152 at level L0 are formed betweenthese Y lines and adjacent X lines 164. In the arrangement of FIG. 2, Xlines 164 are shared between memory layers L0 and L1 and thus, furtherconnect to memory cells 170 at memory level L1. A third set ofconductors form Y lines 174 for these cells at level L1. These Y lines174 are in turn shared between memory levels L1 and memory level L2.Memory cells 178 are connected to Y lines 174 and X lines 176 to formthe third memory level L2. Memory cells 182 are connected to X lines 176and Y lines 180 to form the fourth memory level L3. Memory cells 186 areconnected to Y lines 180 and X lines 184 to form the fifth memory levelL4. The arrangement of the steering element's (e.g., diodes') polarityand the respective arrangement of the X lines and Y lines can vary byembodiment. Additionally, more or less than five memory levels can beused.

If p-i-n diodes are used as steering elements for the memory cells inthe embodiment of FIG. 2, the diodes of memory cells 170 can be formedupside down relative to the p-i-n diodes of the first level of memorycells 152. For example, if cells 152 include a bottom heavily dopedregion that is n-type and a top heavily doped region that is p-type,then in the second level of cells 170, the bottom heavily doped regionmay be p-type while the top heavily doped region is n-type.

In an alternative embodiment, an inter-level dielectric can be formedbetween adjacent memory levels and no conductors are shared betweenmemory levels. This type of structure for three-dimensional monolithicstorage memory is often referred to as a non-mirrored structure. In someembodiments, adjacent memory levels that share conductors and adjacentmemory levels that do not share conductors can be stacked in the samemonolithic three dimensional memory array. In other embodiments, someconductors are shared while others are not. For example, only the Xlines or only the Y lines can be shared in some configurations. Thistype of configuration is often referred to as half-mirrored. Memorylevels need not all be formed having the same type of memory cell. Ifdesired, memory levels using resistive change materials can alternatewith memory levels using other types of memory cells, etc.

In one embodiment as described in U.S. Pat. No. 7,054,219, entitled,“Transistor Layout Configuration for Tight Pitched Memory Array Lines,”word lines are formed using word line segments disposed on differentword line layers of the array. The segments can be connected by avertical connection to form an individual word line. A group of wordlines, each residing on a separate layer and substantiallyvertically-aligned (notwithstanding small lateral offsets on somelayers), may be collectively termed a row. The word lines within a rowpreferably share at least a portion of the row address. Similarly, agroup of bit lines, each residing on a separate layer and substantiallyvertically-aligned (again, notwithstanding small lateral offsets on somelayers), may be collectively termed a column. The bit lines within acolumn preferably share at least a portion of the column address.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as a wafer, withno intervening substrates. The layers forming one memory level aredeposited or grown directly over the layers of an existing level orlevels. In contrast, stacked memories have been constructed by formingmemory levels on separate substrates and adhering the memory levels atopeach other, as in Leedy, U.S. Pat. No. 5,915,167, “Three DimensionalStructure Memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

FIGS. 1 and 2 show memory cells in a pillar shape and conductors in theshape of rails. However, the technology described herein is not limitedto any one specific structure or shape for a memory cell. Otherstructures can also be used to form memory cells that include reversibleresistivity-switching material. For example, the following patentsprovide examples of structures of memory cells that can be adapted touse reversible resistivity-switching material: U.S. Pat. No. 6,952,043;U.S. Pat. No. 6,951,780; U.S. Pat. No. 6,034,882; U.S. Pat. No.6,420,215; U.S. Pat. No. 6,525,953; and U.S. Pat. No. 7,081,377.

FIG. 3 is a block diagram that depicts one example of a memory system300 that can implement the technology described herein. Memory system300 includes a memory array 302 that can be a two or three dimensionalarray of memory cells as described above. In one embodiment, memoryarray 302 is a monolithic three dimensional memory array implemented onone or more integrated circuits. The array terminal lines of memoryarray 302 include the various layer(s) of word lines organized as rows,and the various layer(s) of bit lines organized as columns. However,other orientations can also be implemented.

Memory system 300 includes row control circuitry 320, whose outputs 308are connected to respective word lines of the memory array 302. Rowcontrol circuitry 320 receives a group of M row address signals and oneor more various control signals from System Control Logic circuit 330,and typically may include such circuits as row decoders 322, arrayterminal drivers 324, and block select circuitry 326 for both readingand programming (e.g., SET and RESET) operations. Memory system 300 alsoincludes column control circuitry 310 whose input/outputs 306 areconnected to respective bit lines of the memory array 302. Columncontrol circuitry 306 receives a group of N column address signals andone or more various control signals from System Control Logic 330, andtypically may include such circuits as column decoders 312, arrayterminal receivers or drivers 314, block select circuitry 316, as wellas read/write circuitry, and I/O multiplexers. System control logic 330receives data and commands from a host system (e.g., computer, PDA,telephone, camera, etc.) and provides output data to the host. In otherembodiments, system control logic 330 receives data and commands from aseparate controller circuit and provides output data to that controllercircuit, with the controller circuit communicating with the host. Systemcontrol logic 330 may include one or more state machines, registers andother control logic for controlling the operation of memory system 300.

In one embodiment, all of the components depicted in FIG. 3 are arrangedon a single integrated circuit. For example, system control logic 330,column control circuitry 310 and row control circuitry 320 are formed onthe surface of a substrate and memory array 302 is a monolithic threedimensional memory array formed above the substrate (and, therefore,above system control logic 330, column control circuitry 310 and rowcontrol circuitry 320). In some cases, a portion of the controlcircuitry can be formed on the same layers as some of the memory array.In some embodiments, the memory is made using one or moresemiconductors.

Integrated circuits incorporating a memory array usually subdivide thearray into a number of sub-arrays or blocks. Blocks can be furthergrouped together into bays that contain, for example, 16, 32, or adifferent number of blocks. As frequently used, a sub-array is acontiguous group of memory cells having contiguous word and bit linesgenerally unbroken by decoders, drivers, sense amplifiers, andinput/output circuits. This is done for any of a variety of reasons. Forexample, the signal delays traversing down word lines and bit lineswhich arise from the resistance and the capacitance of such lines (i.e.,the RC delays) may be very significant in a large array. These RC delaysmay be reduced by subdividing a larger array into a group of smallersub-arrays so that the length of each word line and/or each bit line isreduced. As another example, the power associated with accessing a groupof memory cells may dictate an upper limit to the number of memory cellswhich may be accessed simultaneously during a given memory cycle.Consequently, a large memory array is frequently subdivided into smallersub-arrays to decrease the number of memory cells which aresimultaneously accessed. Nonetheless, for ease of description, an arraymay also be used synonymously with sub-array to refer to a contiguousgroup of memory cells having contiguous word and bit lines generallyunbroken by decoders, drivers, sense amplifiers, and input/outputcircuits. An integrated circuit may include one or more than one memoryarray.

Structure

FIG. 4 is a side cut-away view of a portion of a three dimensionalmonolithic memory array, showing two adjacent memory cells. For purposesof this document, adjacent means lying near, close or contiguous. Forexample, it is possible that there could be an intervening layer ofmaterial between two adjacent memory cells. In the embodiment of FIG. 4,the two adjacent memory cells are connected to different Y lines but thesame X line. Note that FIG. 4 only shows one level of the threedimensional monolithic memory array.

The first layer depicted in FIG. 4 is electrode 402. In one embodiment,electrode 402 comprises Titanium Nitride (TiN). Above the electrode 402is conductor 404. In one embodiment, conductor 404 comprises Tungsten(W). In other embodiments, other metals can be used. In one example,conductor 404 corresponds to conductor 12 of FIG. 1 and can be labeledas an X line. Above conductor 404 is electrode 406. In one embodiment,electrode 406 comprises TiN. Above electrode 406 is n+ layer 408. Aboven+ layer 408 is n− layer 410. In one embodiment, n+ layer 408 and n−layer 410 are silicon. In other embodiments, other semiconductors can beused. Electrode 402, conductor 404, electrode 406, and silicon layers408 and 410 are formed in the shape of a rail that runs left to right onthe page depicting FIG. 4. This rail connects to many memory cells.

FIG. 4 shows two adjacent memory cells. For a first memory cell, aboven− layer 410 is p+ layer 420. In one embodiment, p+ layer 420 issilicon; however, in other embodiments other semiconductors can also beused. Above p+ layer 420 is Titanium Disilicide (TiSi₂) layer 422. AboveTiSi₂ layer 422 is electrode 424. In one embodiment, electrode 424comprises TiN. In this embodiment, p+ layer 420, TiSi₂ layer 422 andelectrode 424 are pillar shaped. Above electrode 424 is reversibleresistance-switching material 426. Any of the reversibleresistance-switching materials discussed above can be used. Abovereversible resistance-switching material 426 is electrode 428. In oneembodiment, electrode 428 comprises TiN. Above TiN 428 is conductor 430.In one embodiment, conductor 430 is made of Tungsten (W). In thisembodiment, reversible resistance-switching material 426, electrode 428and conductor 430 are formed as a rail. In the orientation depicted inFIG. 4, the rail of layers 426-430 goes in and out of the page while therail of layers 402-410 extends left to right.

The second memory cell depicted in FIG. 4 includes a p+ layer 440 aboven− layer 410. Above p+ layer 440 is a TiSi₂ layer 442. Above TiSi₂ layer442 is electrode 444. In one embodiment, electrode 444 comprises TiN.Layers 440-444 are pillar shaped, as described above.

Reversible resistance-switching material 446 is above electrode 444.Above reversible resistance-switching material 446 is electrode 448(e.g. TiN). Above electrode 448 is conductor 450, which in oneembodiment is made of Tungsten. In one example, conductor 404 is an Xline and conductors 430 and 450 are Y lines. For example, conductor 404corresponds to conductor 12 of FIG. 1 and conductor 430 corresponds toconductor 10 of FIG. 1.

As discussed above, FIG. 4 only shows one layer of a three dimensionalmonolithic memory array. That memory array would include several layers,each layer being of the structure of FIG. 4. Additionally, each Y lineswill connect to multiple reversible resistance-switching materials in afirst direction (Y direction). The X lines will connect to multiplereversible resistance-switching materials along a second direction (e.g.the X direction). For purposes of this document, the conductors areconsidered to be connected to the reversible resistance-switchingmaterial even though there may be one or more intervening layers ofother materials.

In one embodiment, the pillar layer of FIG. 4 is double etched so thatit is self-aligned with the device rail below and the rail above. TheTungsten conductor line 404 can be formed using a damascene process orit can be an etched line. The electrodes 402 and 406 will be formed withTungsten layer 404. The device rail (comprising n+ silicon layer 408 andn− silicon layer 410) would be a second etching process (after etchinglayers 402-406). When performing a second etch for the device rail(layers 408-410), layers 420-424 and 440-444 are also etched along thesame direction so that these layers initially form a rail similar ingeometry to layers 408 and 410. When forming conductor 430, electrode428 and reversible resistance-switching material 426, a second mask isused and a third etching process is performed. The third etching processetches layers 430, 428, 426, 424, 422, 420 and a small portion ofn-silicon layer 410. This third etching layer forms the rail thatcomprises layers 426-430 and etches layers 420-424 in an orthogonaldirection to the second etch thereby forming layers 420-424 into apillar. Thus, the second etch includes layers 408-424 and the third etchincludes layers 420-430 (with a little portion of layer 410). As such,layers 420-424 are double etched. By extending the double etching belowp+ layer 420 (and p+ layer 440), punch through can be suppressed betweenadjacent pillars.

FIG. 5 depicts another embodiment of a structure for implementingsuitable memory cells for the technology described herein. The structureof FIG. 5 is very similar to the structure of FIG. 4. One differencebetween the structures of FIG. 4 and FIG. 6 is that in the structure ofFIG. 4 the reversible resistance-switching material 426 (and reversibleresistance-switching material 446) is part of the rail that includesconductor 430. In the embodiment of FIG. 5, the reversibleresistance-switching material 427 is part of the pillar that includeslayers 420-424. Similarly, reversible resistance-switching material 447is part of the pillar that includes layers 440-444. Other variations ofthe above-described structures, and other structures, can be used withthe technology described herein.

Note that in the embodiments of FIGS. 4 and 5, the p materials and the nmaterials can be reversed so that p materials are on the rail and nmaterials are in the pillars. Thus, instead of having a PNP structure(p+ 420, n− 410, p+ 440), the device can have a NPN structure. ThusSimilarly, based on whether a PNP or NPN structure is used, currentsidentified herein can be electron currents or hole currents.

Reading

As described above, a reversible resistance-switching element may bereversibly switched between two or more states. For example, thereversible resistivity-switching material may be in an initial,high-resistivity state upon fabrication that is switchable to alow-resistivity state upon application of a first voltage and/orcurrent. Application of a second voltage and/or current may return thereversible resistivity-switching material to a high-resistivity state.FIG. 6 is a graph of voltage versus current for one example embodimentof a metal oxide reversible resistance-switching element. Line 460represents the I-V characteristics of the reversibleresistance-switching element when in the high resistance state(R_(OFF)). Line 462 represents the I-V characteristics of the reversibleresistance-switching element when in the low resistance state (R_(ON)).Vset is the voltage need to SET the reversible resistance-switchingelement to the low resistance state. Vreset is the voltage need to RESETthe reversible resistance-switching element to the high resistancestate.

While in the high-resistivity state (see line 460), if the voltage VSETand sufficient current is applied, the reversible resistance-switchingelement will be SET to the low-resistivity state. When Vset is applied,the voltage will remain somewhat constant and the current will increasetoward Iset_limit. At some point, the reversible resistance-switchingelement will be SET and the device behavior will be based on line 462.Note that the first time the reversible resistance-switching element isSET, Vf (the forming voltage) is needed to SET the device. After that,VSET can be used. The forming voltage Vf may be greater than VSET.

While in the low-resistivity state (see line 462), if the voltage VRESETand sufficient current (Ireset) is applied, the reversibleresistance-switching element will be RESET to the high-resistivitystate.

In one embodiment, Vset is approximately 5 volts, Vreset isapproximately 3 volts, Iset_limit is approximately 5 uA and the Iresetcurrent could be about 30 uA.

To determine which state the reversible resistance-switching element isin, a voltage is applied and the resulting current is measured. A highermeasured current (see line 462) indicates that the reversibleresistance-switching element is in the low-resistivity state. A lowermeasured current (see line 460) indicates that the reversibleresistance-switching element is in the high-resistivity state.

FIG. 6A depicts one embodiment of a circuit used for reading the stateof a memory cell. FIG. 6A shows a portion of a memory array includingmemory cells 470, 472, 474 and 476. Two of the many Y lines and two ofthe many X lines are depicted. A read circuit for one of the Y lines isdepicted to be connected to the Y line via transistor 478, which iscontrolled by a gate voltage supplied by column decoder 312 in order toselect or unselect the corresponding Y line. Transistor 478 connects theY line to a Data bus. Write circuit 484 (which is part of system controllogic 330) is connected to the Data bus. Transistor 482 connects to theData bus and operates as a clamp device that is controlled by clampcontrol circuit 480 (which is part of system control logic 330).Transistor 482 is also connected to comparator 486 and reference currentsupply Iref. The output of comparator 486 is connected to a data outterminal (to system control logic 330, a controller and/or a host) andto data latch 488. Write circuit 484 is also connected to data latch488.

When attempting to read the state of the reversible resistance-switchingelement, all X lines are first biased at Vread (e.g., approximately 2volts) and all Y lines are at ground. The selected X line is then pulledto ground. For example purposes, this discussion will assume that memorycell 470 is selected for reading. One or more selected Y lines arepulled to Vread through the data bus (by turning on transistor 478) andthe clamp device (transistor 482, which receives ˜2 volts+Vt). The clampdevice's gate is above Vread, but controlled to keep the Y line nearVread. Current is pulled by the selected memory cell through transistor482 from the Vsense node. The Vsense node also receives a referencecurrent Iref that is between a high-resistivity state current and alow-resistivity state current. The Vsense node moves corresponding tothe current difference between the cell current and the referencecurrent Iref. Comparator 486 generates a data out signal by comparingthe Vsense voltage to a Vref-read voltage. If the memory cell current islarger than Iref, the memory cell is in the low-resistivity state andthe voltage at Vsense will be lower than Vref. If the memory cellcurrent is smaller than Iref, the memory cell is in the high-resistivitystate and the voltage at Vsense will be higher than Vref. The data outsignal from comparator 486 is latched in data latch 488 and reported tosystem control logic 330, a controller and/or a host.

Forward Bias SET and Reverse Bias RESET

FIG. 7 is a flow chart describing one embodiment of performing a SEToperation, which includes changing the state of the reversibleresistance-switching element to the low-resistance state. The process ofFIG. 7 is performed by row control circuitry 320, column controlcircuitry 310 and system control logic 330. The process of FIG. 7describes how one particular memory cell has its reversibleresistance-switching material SET. In some embodiments, the process ofFIG. 7 can be performed such that multiple memory cells are SETconcurrently. In one embodiment, a host may send data to system controllogic 330. System control logic 330 will then choose a set of memorycells to store that data. As described above, in one embodiment, eachmemory cell is at the intersection of one X line and one Y line.Therefore, after system control logic 330 chooses an address in thememory array 302 to store data, row control circuitry 330 and columncontrol circuitry 310 will select a particular X line and a particular Yline that connect to the chosen memory cell. That X line selected by rowcontrol circuitry 320 is the selected X line, while other X lines areunselected X lines. The Y line selected by column control circuitry 310is the selected Y line, while other Y lines are the unselected Y lines.In step 502, the unselected X lines are biased at a voltage of Vpp-Vt.Vpp is at or near 6 volts. In one embodiment, Vpp is the highest voltageavailable on integrated circuits. In some implementations, theintegrated circuit will receive a power signal that will be supplied toone or more charge pumps and voltage controllers. These charge pumps andvoltage controls will generate a set of voltages, the highest of whichwill be Vpp. In other embodiments, Vpp will not be the highest voltage.In one example, Vpp is also the voltage required to SET the reversibleresistance-switching element to the low-resistance state, plus the diodedrop at the SET current. Vt is an offset voltage equivalent to one diodedrop. In one embodiment, the offset voltage is approximately 0.6 volts.Other offset voltages can also be used. In step 504 the unselected Ylines are biased at the offset voltage (approximately 0.6 volts). Instep 506, the selected X line is biased at ground. In step 508, theselected Y line is biased at a voltage of Vpp.

FIG. 8 is a schematic of a portion of one level of a three dimensionalmonolithic memory array. The schematic shows four memory cells 520, 522,524 and 526. The symbol used for each memory cell includes a symbol fora resistor (representing the reversible resistance-switching element)and a diode (representing a steering device). Note that the diodecorresponds to the p/n junction between p+ layer 420 and n− layer 410 ofFIG. 4. FIG. 8 shows the various biasing described in the method of FIG.7. Note that although FIG. 7 shows four steps in a particular order, thesteps depicted in FIG. 4 can be performed in other orders, includingsome of the steps being performed concurrently.

FIG. 9 is a schematic of two adjacent memory cells where one of thememory cells is being SET according to the process of FIG. 7. FIG. 7shows a selected X line (X line) 578, a selected Y line 580 andunselected Y line 582. In one embodiment, X line 578 of FIG. 8corresponds to Tungsten conductor 404 of FIG. 4, Y line 580 of FIG. 9corresponds to Tungsten conductor 430 of FIG. 4 and Y line 582 of FIG. 9corresponds to Tungsten conductor 450 of FIG. 4. The selected memorycell includes reversible resistance-switching element 586 (whichcorresponds to layer 426 of FIG. 4) and diode 588. In one embodiment,diode 588 represents the pn junction between layer 420 and 410 of FIG.4. The unselected memory cell includes a reversible resistance-switchingelement 590 (corresponding to layer 446 of FIG. 4) and a diode 592corresponding to the pn junction between p+ layer 440 and n− layer 410of FIG. 4. During the SET operation of FIG. 7, current flows from Y line580 through reversible resistance-switching element 586 and diode 588 toX line 578 such that diode 588 is forward biased. The SET current iscontrolled by column control circuitry 310.

Partial etching into the device rail increases the punch through voltagebetween adjacent pillar layers. Optional implant into the device railafter etch of the pillar can also be used to increase punch thoughvoltage. More details about programming in general can be found in U.S.Pat. No. 6,822,903, which is incorporated by reference in its entirety.In one embodiment, before the first target memory cell is SET, it mustbe “formed.” The forming process is typically required for reversibleresistance-switching elements comprising metal oxides. The formingprocess is similar to the SET operation, but with a higher voltage and alonger time. In one embodiment, the forming process can be performedduring manufacturing where temperatures and voltages are controlled totighter specifications than in the user environment.

FIG. 10 is a flow chart describing one embodiment of a process forperforming a RESET operation on a memory cell. In some embodiments, theprocess of FIG. 10 is performed on multiple memory cells concurrentlysuch that multiple cells are RESET at the same time. In otherembodiments, only one memory cell will be RESET at a time. The processof FIG. 10 is performed by column control circuitry 310, row controlcircuitry 320 and system control logic 330.

In step 600 of FIG. 10, a voltage of approximately Vpp-Vt is applied tothe unselected X line. In step 602, unselected Y lines (other than theemitter Y line) are biased between 4 to 5 volts, the same as theselected X line, in order to not cause extra current to flow to theselected X line. In step 604, the selected X line is biased at a voltagebetween 4 and 5 volts. In step 606, the emitter Y line is biased at Vpp.The emitter Y line is a Y line next to the selected Y line that will bea source of current during programming operation. More details of theemitter Y line are provided below. In some embodiments, it may bepreferred that the memory cell connected to the emitter Y line and thesame selected X line is in a conductive state. In step 608, the selectedY line is biased at ground. Note that the unselected X line can also beat a voltage level between Vpp and Vpp-Vt or in a range of 5.5 to 6volts so as not draw current from the emitter Y line.

FIG. 11 is a schematic of a portion of a three dimensional monolithicmemory array showing the biasing of the various lines as described bythe process of FIG. 10. Note that although the process of FIG. 10 showsfive steps being performed sequentially, the steps can be performed inother orders, including some of the steps being performed concurrently.The schematic of FIG. 11 shows the unselected Y line and selected Yline, the emitter Y line, the unselected X line and selected X line. Theschematic of FIG. 11 also shows six memory cells. The first memory cellis depicted by diode 630 and reversible resistance-switching element632. The second memory cell is depicted by diode 634 and reversibleresistance-switching element 636. The third memory cell is depicted bydiode 638 and reversible resistance-switching element 640. The fourthmemory cell is depicted by diode 642 and reversible resistance-switchingelement 644. The fifth memory cell is depicted by diode 646 andreversible resistance-switching element 648. The sixth memory cell isdepicted by diode 650 and reversible resistance-switching element 652.The selected memory cell in this example is the memory cell with diode634 and reversible resistance-switching element 636 because that memorycell is connected to the selected X line and the selected Y line.

With the voltage biases applied to the X lines and Y lines as describedabove in FIGS. 10 and 11, the pn junction for diode 634 (e.g. pnjunction between p+ layer 420 and n− layer 410) is reverse biased. Thus,looking back at FIG. 5, and assuming that Tungsten layer 430 is theselected Y line and Tungsten layer 450 is the emitter Y line; p+ layer420, n− layer 410 and p+ layer 440 act like a bipolar transistor. Thevoltage applied to the emitter Y line (W layer 450), which isapproximately 6 volts, causes a current to flow from the emitter Y linethrough reversible resistance-switching element 640 (e.g. layer 446 ofFIG. 4) through p+ layer 440, into n− layer 410, into p+ layer 420,through reversible resistance-switching material 426 (reversibleresistance-switching element 636 of FIG. 11), and into the selected Yline. This current flow is depicted in FIG. 12 by arrow 682. Thus, theneighboring Y line acts as an emitter line for the memory cell 680,which includes reversible resistance-switching element 636. Because p+layer 420, n− layer 410 and p+ layer 440 are acting like a bipolartransistor, the schematic of FIG. 12 depicts bipolar transistor 686rather than two diodes, in order to show current flow during the RESEToperation.

Reverse Bias SET and Forward Bias RESET

In the above discussion with respect to FIGS. 6-11, the pn junctionbetween p+ layer 420 and n− layer 410 was forward biased for a SEToperation and reverse biased for a RESET operation. In anotherembodiment, depicted by FIG. 13, the pn junction between p+ layer 420and n− layer 410 can be reverse biased for a SET operation and forwardbiased for a RESET operation. For example, FIG. 13 is a schematicdiagram showing a selected Y line, emitter Y line and selected X line.Two memory cells are depicted. A first memory cell is connected to theselected Y line and includes reversible resistance-switching element802. The second memory cell is connected to the emitter Y line andincludes reversible resistance-switching element 804. During a SEToperation of reversible resistance-switching element 802, current flowsas depicted by arrow 810 from emitter Y line to the selected Y linethrough reversible resistance-switching element 804 and reversibleresistance-switching element 802. Looking back at FIG. 4 and assumingthat the selected Y line is Tungsten layer 430 and the emitter Y line isTungsten layer 450, the current would flow from Tungsten layer 450 toreversible resistance-switching material 446, to p+ layer 440, to n−layer 410, to p+ layer 420, to reversible resistance-switching material426, to Tungsten layer 430. With such a current flow, p+ layer 420, n−layer 410 and p+ layer 440 act as a bipolar transistor, as depicted bytransistor 808 in FIG. 13.

When performing a RESET operation, the current flows as depicted byarrow 812 of FIG. 13. That is, during a RESET operation current flowsfrom the selected Y line to the selected X line. Looking back at FIG. 4,the current would flow from Tungsten layer 430 to reversibleresistance-switching material 426, to p+ layer 420, to n− layer 410, ton+ layer 408, to Tungsten layer 404. In the above discussion, the TiNlayers were omitted to shorten the discussion, but the current wouldflow through those layers too.

To allow the SET operation as described with respect to FIG. 13, it ispreferable that the reversible resistance-switching element 804connected to the emitter Y line is in a conductive state, such as thelow-resistance state, prior to trying to performing the SET operationfor the selected memory cell (reversible resistance-switching element802). As mentioned above, there will be many memory cells connected to acommon X line (e.g. the selected X line). Thus, in this embodiment, itis preferable that at least one memory cell on the common X line is inthe conductive stateat all times. The memory cell that is in theconductive state can act as the emitter line in order to RESET anothermemory cell. Then, the newly SET memory cell can act as an emitter for aSET operation for the next memory cell, and so on. In this manner, asequence of SET operations can be performed in a “zipper” fashion. Thatis, for a given set of adjacent memory cells connected to a common Xline, the memory cells can be RESET in consecutive order starting on oneside, thereby providing the analogy of a zipper. This embodimentcontemplates that each X lines will have one (or more than one) memorycell statically in the conductive state. In one implementation, theconductive state is the low resistance state. In another implementation,the conductive state is different than the low resistance state. In someembodiment, the conductive state has a resistance approximately equal tothe resistance of the low-resistance state. In other embodiments, theconductive state has a resistance that is lower than the resistance ofthe low resistance state so that the conducive state is more conductivethan the low resistance state. Such a conductive state may be producedby different pulses than the SET operation described above. The phrasestatically in the conductive state means that the memory cell will stayin the conductive state during the entire operational life of the memoryarray or during a specific period of interest of the operational life ofthe memory array. In one embodiment, the memory cell that is statically(as opposed to able to dynamically change during the period of interest)in the conductive state is permanently in the conductive state.

FIG. 14 is a flow chart describing one embodiment of a process forforming a particular memory cell that is to be permanently in aconductive state. The process of forming puts the reversibleresistance-switching element of that memory cell permanently in aconductive state. This process can be performed by the memory systemupon first operation, during user operation or during the manufacturingphase. In step 830 of FIG. 14, the unselected X lines are biased at 5.5volts. In step 832, the unselected Y lines are biased at 0.7 volts. Instep 834, the selected X line is biased at ground. The memory cell to bepermanently in a conductive state is connected to the selected X lineand the selected Y line. In step 836, high voltage (e.g., approximately9 volts in magnitude) pulses are applied to the Y line connected to thememory cell to be permanently put in a conductive state. In one exampleof the forming operation, element 804 would be a metal oxide layer. Thevoltage, in step 836, is applied for a sufficiently long time to themetal oxide element 804 in order to breakdown the metal oxide element804 to a permanent conductive state in the manner of oxide anti fuses.In one embodiment, the memory cell permanently in a conductive state (orstatically in the conductive state) is referred to as a dummy memorycell because it does not store user data and the Y line connected tothat dummy memory cell is referred to as the dummy Y Line. The steps ofFIG. 14 can be performed in different order than as depicted. In someembodiment, one or more of the steps are performed concurrently.

FIG. 15 is a partial schematic that graphically depicts a portion of amemory array and the various voltages described above in FIG. 14. Thatis, FIG. 15 shows a selected X line, Y line 1, Y line 2, Y line 3, Yline 4, and a dummy Y line. FIG. 15 also shows memory cell 880, 882,884, 886 and 888. Memory cell 880 is connected to Y line 1 and theselected X line. Memory cell 882 is connected to Y line 2 and theselected X line. Memory cell 884 is connected to Y line 3 and theselected X line. Memory cell 886 is connected to Y line 4 and theselected X line. Memory cell 888 is connected to the dummy Y line andthe selected X line. FIG. 15 shows the selected X line biased at groundand a dummy Y line receiving 9 volt pulses. The other Y lines receive0.7 volts. Arrow 890 indicates current flow from the dummy Y line to theselected X line as a result of the voltage biases depicted. As a resultof that current depicted by arrow 804, dummy memory cell 888 isstatically (or permanently) put in the conductive state.

FIG. 16 is a flow chart describing one embodiment of a zipper processthat SETs the memory cells using the dummy memory cell. In step 900, thefirst memory cell will be SET using the dummy memory cell as the emitterline. In step 902, the second memory cell will be SET using the firstmemory cell as the emitter line. In step 904, the third memory cell willbe SET using the second memory cell as the emitter line. In step 906,the fourth memory cell will be SET using the third memory cell as theemitter line. And so on. For example, looking back at FIG. 15, afterforming memory cell 888 to be permanently in the low-resistance state, aSET operation will be performed for memory cell 886, followed by a SEToperation for memory cell 884, followed by a SET operation for memorycell 882, followed by a SET operation for memory cell 880, and so on.

FIG. 17 is a flow chart describing the process for performing a SEToperation on any one of the memory cells connected to the common X line.For example, the process of FIG. 17 can be performed as an exampleimplementation of any one of steps 900-906. The process of FIG. 17 isperformed by column control circuitry 310, row control circuitry 320 andsystem control logic 330.

In step 940 of FIG. 17, the unselected X lines are biased at 5 volts. Instep 942, the unselected Y lines (other than the emitter Y line) receive4.7 volts. In step 944, the selected X line is biased at 4 volts. Instep 946, the emitter Y line is biased at 5.5 volts. If memory cell 886is being set, then the emitter Y line is the dummy Y line. If memorycell 880 is being set, then the emitter Y line is Y line 2. In step 948,the selected Y line is biased to ground. FIG. 18 is a schematic diagramshowing memory cells 880-888 as a result of performing the process ofFIG. 17 during step 900. That is, FIG. 18 shows the memory cells and thevarious voltage levels applied when performing a SET operation for thefirst memory cell 886. As can be seen, dummy Y line is being used as theemitter Y line. Arrow 950 shows the flow of current from the dummy Yline to Y line 1 via memory cell 888 and memory cell 886. Looking backat FIG. 4, the current flows from Tungsten layer 450 to reversibleresistance-switching material 446, to p+ layer 440, to n− layer 410, top+ layer 420, to reversible resistance-switching material 426, toTungsten layer 430. In this manner, p+ layer 440, n− layer 410 and p+layer 420 act like a bipolar transistor.

FIG. 19 is a schematic diagram depicting memory cells 880-888. FIG. 19shows the biasing of the various Y lines and the selected X line as aresult of performing the process of FIG. 17 during step 902 of FIG. 16.As can be seen, arrow 952 depicts the flow of current from Y line 1 to Yline 2 via memory cell 886 and memory cell 884. In this operation,memory cell 886 and Y line 1 serve as the emitter line for memory cell884. With respect to the layers described in FIG. 4, the current flowsin the manner discussed above with respect to FIG. 18.

FIG. 20 is a schematic diagram depicting memory cells 880-888 and thevoltages applied to the various Y lines and the selected X line as aresult of performing the process of FIG. 17 during step 904 of FIG. 16.As can be seen from FIG. 20, Y line 2 and memory cell 884 serve as theemitter line for performing the SET operation for memory cell 882. Thecurrent is depicted by arrow 956 as flowing from Y line 2 to Y line 3via memory cells 884 and 882.

FIG. 21 is a schematic diagram of memory cells 880-888, as well as thevarious Y lines and the selected X line. FIG. 21 shows the voltageapplied to the Y lines and the selected X line as a result of performingthe process of FIG. 17 during step 906 of FIG. 16. As can be seen, Yline 3 and memory cell 882 serve as the emitter line for performing theSET operation for memory cell 880. An arrow 958 represents the currentflow during the SET operation that flows from Y line 3 to Y line 4 viamemory cells 882 and 880. With respect to the layers described in FIG.4, the current flows in the manner discussed above with respect to FIG.18.

FIG. 22 is a flow chart describing one embodiment of a RESET operationperformed on any of memory cells 880-888 (as well as other memory cellsin a three dimensional monolithic memory array) that were SET accordingto the process of FIG. 17. In this process, the RESET operationconfigures the selected cell junction to be forward biased, with nobipolar transistor action. In step 1000, the unselected X lines arebiased at Vpp-offset. In one embodiment, Vt is an offset, as describedabove. In step 1002, the unselected Y line receives ground potential. Instep 1004, the selected X line is biased at ground. In step 1006, thedummy Y line is biased at ground. In one embodiment, each X line willhave one dummy memory cell and all of the dummy memory cells areconnected to the same dummy Y line. In other embodiments, each X linecan include multiple dummy memory cells so that there are multiple dummyY lines. In other embodiments, the dummy memory cells can be connectedto different dummy Y lines. In step 1008, the selected Y lines arebiased at Vpp

FIG. 23 is a schematic drawing of memory cells 880-888 during a RESEToperation for memory cell 884. The dummy Y line and the unselected Ylines receive a ground potential while Y line 3 receives Vpp. Theselected X line (common to memory cells 880-888) also receives ground. Acurrent flows, as represented by arrow 1010, from Y line 3 to theselected X line. This current will cause the memory cell 884 to RESET tothe high-resistance state.

In the above embodiments of FIGS. 13-23, the zipper method was used toSET the memory cell. In an alternative embodiment, the zipper method canbe used with respect to the embodiment of FIGS. 6-11 to RESET the memorycell. In such an embodiment, there will still be a need for one dummymemory cell statically in a conductive state.

Multi-Collector Memory Cell

One embodiment uses multiple reversible resistance-switching elements tocreate multi-bit memory cells. That is, in the discussion above, eachmemory cell included one resistance-switching element that could be inone of two states. Therefore, each memory cell stores one bit of data.In other embodiments, one reversible resistance-switching element can beput into any one of four or eight resistance states. In that case, thereversible resistance-switching element can store two or three (or more)bits of data. In another embodiment, multi-bit memory cells can beachieved using more than one reversible resistance-switching element. Inone example, a memory cell includes multiple reversibleresistance-switching elements that can each be in the high or lowresistance state (or more than two states).

FIG. 24 is a schematic drawing showing one example of a multi-bit memorycell that uses multiple reversible resistance-switching elements.Another name for a memory cell with multiple reversibleresistance-switching elements is a multi-collector memory cell sinceeach of the reversible resistance-switching elements that can store userdata can serve as a collector (for the bipolar transistor actiondescribed herein) during the SET operation for that reversibleresistance-switching element. The multi-bit memory cell of FIG. 24 showsthree resistance elements 1100, 1102 and 1104. In one embodiment,elements 1100 and 1104 are reversible resistance-switching elements, andelement 1102 is statically in a conductive state, such as the conductivestate described above. Element 1102 is referred to as a staticresistance element because its resistance is not switchable duringnormal user operation of the memory device. Statis resistance element1102 can be material that is always in a conductive state or reversibleresistance-switching material that is statically put in a conductivestate. The Y line for the statis resistance element that is staticallyin the conductive state is referred to as the Emitter line because itserves as the emitter when the p, n and p materials that act like atransistor, as discussed herein.

FIG. 24 shows a reversible resistance-switching element 1100 connectedbetween a first Y line and the common X line, and reversibleresistance-switching element 1104 connected between the second Y lineand the common X line. When either reversible resistance-switchingelement 1100 or 1104 are RESET to the high-resistance state, currentwill flow from the respective Y line to the common X line. For example,arrow 1110 shows current being passed from the first Y line to thecommon X line when performing a RESET operation for memory element 1100.When either memory element is SET, current will flow between two Y linesof the memory cell. In one example, when the first reversibleresistance-switching element is SET, current will flow from the Y lineconnected to the static resistance element to the Y line connected tothe reversible resistance-switching element being SET. When the secondreversible resistance-switching element is SET, current will flow fromthe Y line connected to the static resistance element to the Y lineconnected to the second reversible resistance-switching element beingSET. In another embodiment, when the second reversibleresistance-switching element is SET current will flow from the Y lineconnected to the first reversible resistance-switching element that wasalready SET to the Y line connected to the second reversibleresistance-switching element being SET. FIG. 24 shows arrow 1108representing current from the emitter Y line (the Y line for staticresistance element 1102) to the first Y line.

Each of the two reversible resistance-switching elements can either bein the high-resistance state or the low-resistance state; therefore, thememory cell as a whole can be in four different data states asrepresented by the table below:

Reversible Resistance- Reversible Resistance- Switching Element 1100Switching Element 1104 Data State A H H Data State B H L Data State C LH Data State D L L

FIG. 25 is a schematic diagram showing a portion of one level of a threedimensional monolithic memory array. The schematic of FIG. 25 showselements 1130, 1132, 1134, 1136, 1138, 1140, 1142, 1144, 1146, 1148,1150, 1152, 1154 and 1156. FIG. 25 shows portions of two X lines (X1 andX2) and portions of seven Y lines (Y1, Y2, Y3, Y4, Y5, E1 and E2). It iscontemplated that along an X line, there are much more reversibleresistance-switching elements than those depicted in FIG. 25. Thevarious elements along an X line are grouped together to form themulti-bit memory cells. For example, FIG. 25 shows four memory cells1160, 1162, 1164 and 1166. Memory cell 1160 includes reversibleresistance-switching element 1130, reversible resistance-switchingelement 1134, and static resistance element 1132. Memory cell 1162includes reversible resistance-switching element 1136, reversibleresistance-switching element 1140, and static resistance element 1138.Memory cell 1164 includes reversible resistance-switching element 1144,reversible resistance-switching element 1148, and static resistanceelement 1146. Memory cell 1166 includes reversible resistance-switchingelement 1150, reversible resistance-switching element 1154, and staticresistance element 1152.

The memory cells in FIGS. 24 and 25 include two reversibleresistance-switching elements that can store user data and one staticresistance element. In other embodiments, more than two reversibleresistance-switching elements can be used. For example, the memory cellof FIG. 26 includes four reversible resistance-switching elementsincluding reversible resistance-switching element 1200 connected betweenY line A and the common X line, reversible resistance-switching element1202 connected between Y line B and the common X line, reversibleresistance-switching element 1206 connected between Y line C and thecommon X line, reversible resistance-switching element 1208 connectedbetween Y line D and the common X line and static resistance element1204 connected between the emitter Y line and the common X line. Otherembodiments can include three reversible resistance-switching elementsfor storing user data or more than four reversible resistance-switchingelements for storing user data.

FIG. 27 is a flow chart describing one embodiment of a process foroperating a memory array with multi-bit memory cells as discussed abovewith respect to FIGS. 24-26. In step 1250 of FIG. 27, the emitters ofeach memory cell will be formed. As discussed above, in one embodiment,one of the resistance elements in each memory cell will be dedicated asa static resistance element in a conductive state. In step 1250, thatmemory cell dedicated to be in the conductive state will be formed to bein that conductive state either permanently or semi-permanently. In step1252, the other reversible resistance-switching elements will berandomly SET and RESET (separately or concurrently) based on data beingstored by a user of the memory system. There is a dotted line betweensteps 1250 and 1252 to indicate that a random and unpredictable amountof time can occur between steps 1250 and 1252.

FIG. 28 is a flow chart describing a process for performing anotherembodiment of operating a three dimensional memory array with multi-bitmemory cells. In step 1256, the emitters for each memory cell areformed. In step 1258, the system (see FIG. 3) will receive data from ahost (or other device). There is a dotted line between steps 1256 and1258 to indicate that a random and unpredictable amount of time canoccur between steps 1256 and 1258. The data received from the host isreceived by system control logic 330 (or another element). In step 1260,system control logic 330 will assign multiple bits of data to eachmemory cell of a set of multi-bit memory cells. In step 1262, all thememory cells to be programmed will have each reversibleresistance-switching element RESET to the high-resistance state. In oneembodiment, memory cells can be programmed on a block by block basis,bay by bay basis, X line by X line basis, page by page basis, or otherunit of programming. At the end of step 1262, the unit of programmingincludes memory cells that have all of their reversibleresistance-switching elements RESET to the high-resistance state. Atthis point, the various memory cells will be programmed into any of thefour states described above in the table above. In step 1264, the firstbit of a subset of memory cells to be programmed are SET to thelow-resistance state. More than one memory cell can be programmed at thesame time. Because some memory cells will store different data, not allthe memory cells will have their first bit SET. For example, if eachmemory cell has a first bit and second bit, some of the memory cellswill have their first bit SET to the low-resistance state and otherswill keep their first bit in the high-resistance state so that some ofthe bits can store 1 and some of the bits can store data 0. In step1266, the second bit for a subset of memory cells is SET to the lowresistance state. Multiple memory cells can have their bits setconcurrently or they can be done consecutively. Again, some of thememory cells will keep their second reversible resistance-switchingelements in the high-resistance state and others will SET the secondreversible resistance-switching element to the low-resistance state sothat different memory cells will store data 1 or data 0. In step 1268,it is determined whether there is any more data to be programmed. Ifnot, the process is done. If so, the process loops back to step 1264 andmore memory cells are programmed. In some embodiments, only a smallnumber of memory cells can be concurrently programmed. Therefore, theloop of steps 1264 and 1266 must be repeated for each group of memorycells that can be concurrently programmed.

FIG. 29 is a flow chart describing one embodiment for forming the staticresistance elements. The process of FIG. 29 can be used to implementsteps 1250 of FIG. 27 or 1256 of FIG. 28. In step 1270, the unselected Xlines are biased at 5.5 volts. In step 1272, unselected Y lines arebiased at 1 volt. In step 1274, the selected X line is placed at ground.Again, the selected X line and selected Y line are those lines thatconnect to the reversible resistance-switching element that will serveas the emitter (sometimes known as the dummy). In step 1276, the emitterY line is biased at 6 volts. As a result of the process of FIG. 29, oneor more static resistance elements are formed to be in the conductivestate.

FIG. 30 is a schematic depicting the performance of the process of FIG.29. FIG. 30 shows three resistance elements 1100, 1102 and 1104, asdescribed above. Resistance element 1102 is being formed to become thestatic resistance element (also referred to as emitter or dummy). Basedon the voltages applied by the process of FIG. 29, a current is passedfrom the emitter Y line to the common X line, as indicated by arrow1290.

FIG. 31 is a flow chart describing one embodiment of a process ofperforming a SET operation to set one of the multiple reversibleresistance-switching elements of a multi-bit memory cell as describedabove. During the SET operation, the pn junction described above isreverse biased so that adjacent pn junctions operate like a bipolartransistor, as described above. That is, looking at FIG. 4, p+ layer420, n− layer 410 and p+ layer 440 operate as a bipolar transistor.

In step 1300 of FIG. 31, unselected X lines are biased at 5.0 volts. Instep 1302, the unselected Y line receives 4.7 volts. In step 1304, theselected X line is biased at 4 volts. In step 1306, the emitter line isbiased at 5.5 volts. In some embodiments, multiple memory cells will beprogrammed at the same time so that multiple emitter lines will bebiased in step 1306. In step 1308, the selected Y line is set at ground.If both reversible resistance-switching elements of a memory cell are tobe SET, the process of FIG. 31 will be performed twice. FIG. 32A showschematics of a memory cell when a first reversible resistance-switchingelement of a memory cell is programmed. FIG. 32A shows the Y lines andselected X line at the biases described above with respect to FIG. 31.As a result of these voltages, a current (identified by arrow 1340)flows from the emitter Y line to Y line 1 via the static resistanceelement 1102 and the reversible resistance-switching element 1100, asper the bipolar transistor action described above.

FIG. 32B is a schematic diagram showing the multi-bit memory cell beingprogrammed to SET the second reversible resistance-switching element1104. There are two potential embodiments. In the first embodiment,current flows from Y line 1 to Y line 2 (via reversibleresistance-switching elements 1100 and 1104), as identified by arrow1342 a. In another embodiment, current flows from the emitter Y line toY line 2 (via static resistance element 1102 and reversibleresistance-switching element 1104). In both cases, the bipolartransistor action described above takes place and the pn junction of thereversible resistance-switching element being SET is reverse biased.

FIG. 33 is a flow chart describing one embodiment of a process forperforming a RESET operation on the multi-bit memory cell describedabove. In step 1400, all unselected X lines are biased at Vpp-offset. Inone example, the offset is Vt that is described above. In step 1402, theunselected Y lines are set to ground. In step 1404, the selected X lineis set to ground. In step 1406, the emitter Y line is set to ground. Insome embodiments, where there are more than one memory cell beingprogrammed at the same time, multiple emitter Y lines can be set toground. In step 1408, the selected Y line is set at Vpp.

FIGS. 34A and 34B are schematic diagrams depicting the performance ofthe process of FIG. 33. For multi-bit memory cells to have multiplecollectors, each collector will be RESET separately. For example, FIG.34 shows a first reversible resistance-switching element being RESET.FIG. 34B shows the second reversible resistance-switching element of amulti-bit memory cell being RESET. Both schematics show the variousvoltages being applied to the Y lines and selected X line. FIG. 34Ashows a current identified by arrow 1450 passing from the selected Yline for the first reversible resistance-switching element 1100 to theselected X line. FIG. 34B shows a current represented by arrow 1452passing from the selected Y line for reversible resistance-switchingelement 1104 to the selected X line. This current is used to RESET thereversible resistance-switching element.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

What is claimed is:
 1. A method of programming a non-volatile storagesystem, comprising: programming multiple sets of data into differentmulti-bit memory cells of a plurality of multi-bit memory cells, eachset of data includes more than one bit of data, each multi-bit memorycell includes a resistance element in a conductive condition and two ormore reversible resistance-switching elements, the resistance element inthe conductive condition and the two or more reversibleresistance-switching elements are connected to different Y lines, theresistance element in the conductive condition and the two or morereversible resistance-switching elements are connected to a common Xline, the common X line is connected to multiple multi-bit memory cells,the different Y lines are connected to multiple multi-bit memory cells,for at least a subset of the multi-bit memory cells the programmingcomprises applying a selected bias to a selected Y line of the differentY lines such that the selected bias is provided to multiple multi-bitmemory cells, applying a bias to the common X line connected to multiplemulti-bit memory cells independent of the applying the bias to theselected Y line such that the bias applied to the common X line isprovided to multiple multi-bit memory cells, applying an unselected biasto unselected Y lines of the different Y lines to cause a first currentto pass from the selected Y line to an unselected Y line though theresistance element in the conductive condition and one of the reversibleresistance-switching elements.
 2. The method of 1, wherein: theresistance element in the conductive condition is statically in aconductive state for a remaining operational life of the non-volatilestorage system.
 3. The method of 1, wherein: the multi-bit memory cellseach have two reversible resistance-switching elements; each reversibleresistance-switching element stores one bit of data; and the programmingmultiple sets of data into different multi-bit memory cells includescausing one or both of the two reversible resistance-switching elementsto change state to a low resistance state.
 4. The method if claim 1,further comprising: receiving the multiple sets of data from a hostdevice, the programming causes a subset of the reversibleresistance-switching elements to change from a high resistance state toa low resistance state.
 5. The method of 1, wherein: for each multi-bitmemory cell, the resistance element in the conductive condition isconnected to a first Y line; each multi-bit memory cell has a firstreversible resistance-switching element of the two or more reversibleresistance-switching elements connected to a second Y line and a secondreversible resistance-switching element of the two or more reversibleresistance-switching elements connected to a third Y line; and theprogramming includes passing the first current between the first Y lineand the second Y line via the resistance element in the conductivecondition to change the first reversible resistance-switching elementfrom a first state to a second state.
 6. The method of claim 5, wherein:the programming further includes passing a second current between thesecond Y line and the third Y line via the first reversibleresistance-switching element to change the second reversibleresistance-switching element from the first state to the second state.7. The method of claim 5, further comprising: changing the firstreversible resistance-switching element from the second state to thefirst state by passing a current between the second Y line and thecommon X line.
 8. The method of claim 5, wherein: multiple multi-bitmemory cells share a semiconductor region of a first type; eachmulti-bit memory cell further includes semiconductor regions of a secondtype in series with reversible resistance-switching elements; and theprogramming includes biasing the Y lines and the common X line to causethe semiconductor region of the first type and the semiconductor regionsof the second type to operate like transistors.
 9. A method ofprogramming a non-volatile storage system, comprising: programmingmultiple bits of data into a single memory cell, the single memory cellincludes a resistance element statically in a conductive condition thatis connected to a particular Y line and a common X line, the singlememory cell includes a first reversible resistance-switching elementconnected to a first Y line and a second reversible resistance-switchingelement connected to a second Y line, the first reversibleresistance-switching element and the second reversibleresistance-switching element are connected to the common X line, thecommon X line is also connected to other memory cells, the programmingcomprises a first operation that includes applying a bias to the commonX line that is also connected to other memory cells so that the bias isreceived at the other memory cells and passing a first current betweenthe particular Y line and the first Y line to cause the first reversibleresistance-switching element to change state to a low resistance state,the first current passes through the resistance element in theconductive condition and the first reversible resistance-switchingelement.
 10. The method of claim 9, wherein: the programming furthercomprises a second operation that includes passing a second currentbetween the particular Y line and the second Y line to cause the secondreversible resistance-switching element to change state to the lowresistance state.
 11. The method of claim 9, further comprising: passinganother current between the first Y line and the common X line, via thefirst reversible resistance-switching element, to cause the firstreversible resistance-switching element to change state from the lowresistance state to a high resistance state.